Optical switches can be used in a variety of applications, such as optical fiber transmission networks, to route optical signals along various signal paths. An optical switch typically has an optical element, such as a mirror or a filter, that is switched into and out of a path of an optical signal beam. Switches are typically characterized by the number of input and output port, referred to as N.times.N. For example, a 1.times.2 switch would switch one input between two outputs.
Switches can often be described as "latching" or "non-latching". A latching switch reliably remains in a known position, even if the power is removed or lost. A non-latching switch may revert to an unknown position, or even a position intermediate between switch states, when the power is lost, for example if current provided to an electro-magnetic solenoid or thermal actuator is lost. One type of latching switch reverts to a known default position (state), no matter what state the switch was in when power was lost. Another type of latching switch preserves the switch state, no matter what that state was. The latter case is known as a "bi-stable" switch.
Bi-stable optical switches are desirable for use in optical telecommunication systems because they preserve the network configuration associated with the position of the switch(es) when the power was lost. Various approaches have been used to produce bi-stable optical switches. One approach uses a permanent magnet in conjunction with a piece of magnetic material to hold the switch element in the desired position. Other approaches use a mechanical latch to hold the switch element in the desired position.
In a particular application, as illustrated and described in U.S. Pat. No. 5,994,816 entitled THERMAL ARCHED BEAM MICROELECTROMECHANICAL DEVICES AND ASSOCIATED FABRICATION METHODS by Dhuler et al., issued Nov. 30, 1999, a mechanical latch is used in a micro-electro-mechanical system ("MEMS") (See, e.g. FIG. 11, ref. nums. 69 and 68c). A thermal arched beam actuator is used to move a switch element back and, with a thermally activated latch holding the switch element in the desired position(s). However, having contact surfaces between the latch and the switch element can result in the mechanism sticking or produces "sticktion (i.e. sticking friction), thus altering the force required to change switch states. This sticking or sticktion can not only affect the reliability of switch operation, but also affect the timing of the switch, particularly with fast (i.e. .ltoreq.1 ms) in light of the need to time the operation of the latch with the operation of the electrostatic comb.
U.S. Pat. No. 5,994,816 also describes a latching mechanism that uses an electrostatic field to clamp a movable portion of the switch to the switch body (substrate). Clamping allows the relatively high current flow to the thermal beam actuator to be removed without losing the clamped switched state, thus conserving power. However, if the voltage to the electrostatic clamping circuit is removed the switch may revert to a state other than what was previously held.
Accordingly, it is desirable to provide a bi-stable MEMS switch without mechanical contacting surfaces between moving and static surfaces of the switch. It is further desirable that the optical switch be repeatable and have a high switching lifetime, and maintain a present switch state when power to the switch is removed.